Broad-area irradiation of small near-field targets using ultrasound

ABSTRACT

An ultrasonic transducer system for treating a portion of tissue. The ultrasonic transducer system includes a frequency generator, an ultrasonic transducer, and a transmitter element. The transducer receives an AC voltage from the frequency generator and produces an ultrasonic energy pulse at an ultrasonic frequency for a pulse width. The transmitter element is coupled to the transducer and is for irradiating a portion of skin tissue. The transmitter element has a chilled surface in contact with the skin tissue and an acoustic aperture for producing a substantially collimated energy beam. The beam has a width greater than 4 mm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 12/642,686, filedDec. 18, 2009, which in turn claims priority to U.S. ProvisionalApplication Ser. No. 61/139,813 filed Dec. 22, 2008, both of which areincorporated herein by reference in their entirety for all purposes.

FIELD OF THE INVENTION

This application generally relates to broad-area irradiation of smalltargets within an irradiation area. More specifically, this applicationrelates to the use of ultrasound irradiation to inhibit hair growth inskin tissue.

BACKGROUND

Ultrasound systems have a wide range of applications including, forexample, medical procedures for imaging, diagnosis, or treatment of ahuman body. Using an ultrasonic transducer, energy can be transmitted toadjacent tissue so that the energy can be absorbed by parts of the body.

The concept of using ultrasound radiation to remove unwanted hairappeared in the literature as early as 1998. (Iger et al., WO 00/21612“A Method and Device for Hair Removal,” “Iger” hereinafter.) Theunderlying principal is to use ultrasound radiation to selectivelyinduce damage to the hair structure and thereby retard its ability toregenerate. Typically, the bulb or bulge of the hair follicle istargeted since these features are thought to be involved in theregenerative process of hair growth. These features are commonly locatedseveral millimeters below the skin surface.

To date, two main techniques have been proposed for delivering theradiation to the hair follicle. In one, the hair shaft is gripped abovethe skin by some mechanical means and radiation is then coupled directlyinto either the side or end of the hair shaft. (See, e.g., Barzilay etal., U.S. Patent Application 2007/0173746 “Method and Device forRemoving Hair,” “Barzilay” hereinafter.) An ultrasonic transducer thenfocuses the radiation into the shaft, which transmits the energy to thehair follicle below. Alternatively, the radiation may be focused throughthe skin to a point of high intensity on a hair follicle. (See, e.g.,Masotti., WO 02/09813 “Method and Device for Epilation by Ultrasound,”“Masotti” hereinafter.) Since the targeted location is typically severalmillimeters below the skin surface, the practical limits of beamfocusing requires that the beam radius on the skin surface to be lessthan several millimeters wide.

These beam delivery methods are similar in that they both use a form ofspatial selectivity to concentrate the radiation within the hairstructure, and thereby damage it, without affecting the surroundingtissues. In addition, both techniques focus the beam onto the hair togenerate the intensity required to create damage. Two advantages of thisapproach are: 1) the target is located in the far-field of thetransducer (close to the focal plane) where the beam's intensity profilehas a smooth shape; 2) a low-power transducer is required since theoutput intensity at the transducer is significantly lower before it isfocused. However, the inherent spatial selectivity prevents theapplication of these techniques to treating many hairs simultaneously.In particular, the wide variability in the spacing, angle, and length ofhair shafts makes it impractical to grab, position, and efficientlyirradiate a large number of hairs at one time. Because the spacing ofthe hairs may vary slightly, it is also impractical to design a devicewith multiple focal points aligned to individual follicles.

This lack of scalability makes these techniques unsuitable alternativesto existing light-based technologies that are capable of treating largeareas in a short period of time. For example, common areas forlight-based hair removal treatments include the axilla (armpit), arms,legs, back, chin, and pubic areas where the hair density ranges from 50to 500 follicles/cm². (Helen R. Bickmore, Milady's Hair RemovalTechniques: A Comprehensive Manual, Thompson Learning Inc. (2004).)Using light-based technologies, the typical treatment area may rangefrom 1 to more than 100 cm², and the treatments can be performed atspeeds up to 3 cm²/sec. As a result, using light-based technologies, 50to 50,000 hairs may be treated in a period between 1 and 33 sec.

What's needed is an ultrasonic device that can deliver performancecomparable to existing light-based techniques. Specifically, there is aneed for an ultrasonic device that can treat multiple hairs using awide-area exposure. Preferably, a device should have an effectivetreatment area of about 1 mm² or greater since this would allowtreatment of at least 5 hairs at one time.

SUMMARY

In one aspect of the present invention, an ultrasonic transducer systemfor treating tissue comprises: a frequency generator, a transducer, anda transmitter element. The frequency generator generates an AC voltage.The transducer receives the AC voltage from the frequency generator andproduces an ultrasonic energy pulse at an ultrasonic frequency for apulse width. The transmitter element is coupled to the transducer andirradiates a portion of skin tissue. The transmitter element has achilled surface in contact with the skin tissue. The transducer ortransmitter has an acoustic aperture for producing a substantiallycollimated energy beam. The substantially collimated energy beam has awidth greater than 4 mm.

In some aspects, the transmitter element is comprised of at least onelayer with a thickness and an acoustic impedance. The thickness andacoustic impedance of the transmitter element are selected so that atleast 50% of the ultrasonic energy pulse is transmitted into the skintissue.

In some aspects, the ultrasonic energy pulse has an acoustic wavelength,and the square of half the acoustic aperture, divided by the product ofthe acoustic wavelength and the distance from the acoustic aperture to 5mm below the skin surface, is greater than 10.

In some aspects, the intensity of the ultrasonic energy pulse is greaterthan or equal to 150 W/cm, the ultrasonic frequency is between 5 and 20MHz, and the pulse width is less than 100 milliseconds.

In some aspects, the acoustic aperture has a non-circular shape. In someaspects, the acoustic aperture may have a quadrilateral shape or anelliptical shape. In some aspects, the acoustic aperture has a roundedshape with one of a serrated, rippled, notched, or jagged edge profile.

In some aspects, the transmitter element includes an apodizing annulusfor attenuating the transmitted power near the perimeter of the energybeam. The intensity of the transmitted power near the perimeter of theenergy beam is within 20% of the intensity of the transmitted power nearthe center of the energy beam.

Some aspects of the present invention include a method of inhibitinghair growth in a skin tissue. The method includes placing an ultrasonictransducer in contact with the skin tissue. The transducer has anacoustic aperture for producing a substantially collimated energy beamover a treatment area greater than 16 mm². The transducer also has achilled surface in contact with the skin tissue. The tissue has aplurality of hair follicles within the treatment area. The methodfurther includes generating one or more ultrasonic energy pulses, eachenergy pulse at a corresponding frequency and pulse width.

DESCRIPTION OF THE FIGURES

FIG. 1 depicts an embodiment of a transducer with an impedance-matchingchill plate.

FIG. 2 depicts an intensity profile of a 10 MHz source 2 mm from a2-mm-wide aperture.

FIG. 3 depicts intensity profiles for a 10 MHz source 2 mm fromapertures with widths of 2, 4, and 20 mm.

FIG. 4 depicts an intensity profile for a focused beam.

FIG. 5 depicts a comparison of normalized intensity distributions at 10MHz.

FIG. 6 a depicts an intensity profile for a 2-mm square aperture.

FIG. 6 b depicts an intensity profile for a 2-mm circular aperture.

FIGS. 7 a and 7 b depict examples of rounded aperture shapes withirregular edges.

FIG. 8 depicts an air-backed piezo transducer with an apodizing annulus.

FIG. 9 depicts an example of reflection reduction using stepped andgraded apodization.

FIG. 10 depicts an intensity profile with and without apodization.

FIGS. 11 a-e depict thermal images used to map a beam's transverseintensity profile created using a 20-mm wide transducer operated at (a)10.72, (b) 10.77, (c) 10.82, (d) 10.87, and (e) 10.92 MHz using a 3-mmthick piece of tissue mimicking material.

FIG. 12 depicts a superposition of thermal images used to map a beam'stransverse intensity profile using a 20-mm-wide transducer operated at10.72, 10.77, 10.82, 10.87, and 10.92 MHz for pulse durations of 5, 8,5, 8, and 4 ms, respectively.

FIG. 13 a depicts a comparison of calculated and measured intensityprofiles using two frequencies with 2:1 weighting.

FIG. 13 b depicts a comparison of calculated and measured intensityprofiles using six frequencies with 1:1:2:7:4:3 weighting.

FIG. 14 a depicts a single-element transducer with input waveform topositive electrode.

FIG. 14 b depicts a multi-element transducer with each element beingdriven with a different energy pulse.

FIG. 15 depicts an exemplary longitudinal energy distribution.

FIG. 16 depicts an example of beam imaging.

FIG. 17 depicts an exemplary system using an ultrasonic device.

The figures depict one embodiment of the present invention for purposesof illustration only. One skilled in the art will readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein can be employed without departing fromthe principles of the invention described herein.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

In some embodiments, an ultrasonic transducer is used to treat to aportion of tissue. In the field of dermatology, skin tissue may betreated using irradiation produced by an ultrasonic transducer placed incontact with the surface of the skin. An ultrasonic transducer is usefulfor performing a variety of treatments including, for example, skinlaxity, skin wrinkles, and skin hair removal. In a preferred embodiment,skin hair may be treated using an ultrasonic transducer. In the case ofhair reduction by ultrasound, the goal is to damage the hair follicle byultrasound-induced thermal or mechanical effects

In general, an ultrasonic device may be characterized as a devicecapable of producing displacements at a frequency higher than theaudible range of a human ear (frequencies>20,000). Ultrasonic devicestypically include a transducer that converts electrical energy intoacoustical energy via vibrational motion at ultrasonic frequencies. Theultrasonic vibration is induced by exciting one or more piezoelectric(“piezo”) elements of the transducer using an electrical signal. In apreferred embodiment, a high-frequency electrical signal is transmittedto a pair of electrodes coupled to one or more piezoelectric elements,whereby an electric field is established across the one or morepiezoelectric elements. The electric field generates a mechanicalstanding wave at a frequency approximately equal to the frequency of theelectrical signal. The mechanical standing wave is able to transmitacoustic energy through a medium. In a preferred embodiment, the piezois mechanically coupled to a transmitter mass designed transmit theacoustic energy to a portion of the body.

1. Broad-Area Ultrasonic Transducer Device

To facilitate treatment, an ultrasonic transducer may have an applicatorsurface which is placed in contact with a portion of the body, hereinreferred to as “tissue.” In one embodiment, the transducer produces abroad, unfocused (or weakly focused) beam onto an area of skin tissuesuch that many hairs are within the beam cross-section and may beirradiated. In contrast to the focused beam approach discussed above,the area of the broad beam remains roughly constant as the beampropagates from the skin surface to the depth of a hair follicle (up to7 mm). Within the broad beam there is no spatial selectivity allowingthe device to treat multiple hairs at a time. Also, the intensity of theradiation is nominally the same at the hair follicle as it is in theintervening tissues. Instead of the spatial selectivity of previousultrasonic methods, a broad-area beam achieves selectivity by leveragingthe stronger ultrasound absorption of hair follicles compared to thesurrounding soft tissues.

A goal of a preferred embodiment is to optimize the amount of energyabsorbed by the hair follicle without damaging surrounding tissue. Theenergy absorbed in the hair follicle induces a temperature rise in thebulb or bulge of the follicle, which is believed to provide an effectivetreatment for hair removal.

In preferred embodiments, a device uses beam frequencies between 5 and20 MHz. In a more preferred embodiment, a device uses beam frequenciesbetween 7 and 15 MHz. In preferred embodiments, a device uses pulsedurations between 5 and 100 ms. In a more preferable embodiment, thepulse duration is between 5 and 75 ms. In the most preferableembodiment, the pulse duration is between 5 and 50 ms. In preferredembodiments, the beam area is greater than or equal to 4 mm². In a morepreferable embodiment, the beam area is greater than or equal to 16 mm².In the most preferable embodiment, the beam area is greater than orequal to 50 mm². In preferred embodiments, the beam intensity is greaterthan or equal to 150 W/cm². In a more preferable embodiment, the beamintensity is greater than or equal to 300 W/cm².

In some embodiments, the ultrasonic transducer is used in a single pulse“stamping mode.” For example, the ultrasonic transducer may be placed ata first location covering a first area of tissue. The transducerirradiates the first area of tissue with an ultrasonic pulse.

The transducer may then be placed at a second location covering a secondarea of tissue. The transducer irradiates the second area of tissue withan ultrasonic pulse. In some embodiments, the first and second areas aretiled to minimize overlap or gaps between areas of irradiated tissue. Insome embodiments, the transducer may be used in a “gliding mode,” wherethe transducer is moved in a continuous fashion. In some embodiments,the single ultrasonic pulse consists of multiple frequencies. Thefrequencies may be applied simultaneously or as a series of shortpulses. Using a broad-area transducer (e.g., producing an ultrasonicbeam larger than 4 mm wide) it may take approximately 10 to 30 minutesto treat a leg, and approximately 30 to 60 seconds to treat a chin.

In a preferred embodiment, a treatment protocol includes one pulse every6 or less seconds, each pulse duration being less than 100 ms. In a morepreferable embodiment, a treatment protocol includes one pulse every 3or less seconds, each pulse duration being less than 100 ms.

By allowing multiple hairs to be treated within a single treatment area,a broad-area irradiation device provides a distinct advantage overfocused beam methods. However, there are problems, unique to broad-areatransducers, that are addressed by various aspects of the embodimentsdescribed herein.

A first consideration is that the peak power of the transducer activeelement may be much higher as compared to other medical ultrasounddevices. Broad-area transducers may be characterized as having anunfocused or weakly focused energy beam. As a result, the transmittedenergy in an unfocused beam is distributed over a larger area than thatof a focused beam. The intensity of the beam may be increased to achievethe desired exposure level in the hair follicles. Depending on thefrequency and pulse duration used, the peak intensity energy required atthe hair follicle to achieve hair removal may be between 150 and 1000W/cm². Because the beam is collimated, this energy is also the outputintensity required at the active element of the transducer (e.g.,piezoelectric disk). In comparison, the highest power focused ultrasoundtransducers typically produce intensities up to 4 W/cm² at the activeelement and transducers used for physiotherapy and diagnosticapplication produce up to 0.2 and 5 W/cm², respectively. (C. R. Hill etal. Physical Principles of Medical Ultrasonics, 2^(nd) edition, JohnWiley & Sons Ltd (2004).)

A second consideration is that an increase in ultrasonic power mayproduce more thermal energy, increasing the risk that the tissue beingtreated will be overheated or damaged. In some embodiments, cooling maybe provided to prevent the tissue (e.g., epidermis tissue) fromoverheating. For example, the cooling may be provided by contacting achilled surface to the skin tissue, commonly referred to as a “chillplate.” Cooling may be applied before (“pre-cooling”) or after(“post-cooling”) the exposure or by some combination thereof. In someembodiments, the chill plate temperature may be set between 5 and 30degrees Celsius and preferably between 5 and 20 degrees Celsius. In someembodiments, the cooling period is 0.5 seconds or longer. In general,the cooling period depends on multiple factors including the chill platetemperature, thermal conductance of the chill plate, and the fluence ofthe exposure.

The transducer should also include a mass acting as animpedance-matching element which matches the acoustic impedance of thetransducer active element (typically a piezoelectric disk) to theimpedance of the skin. Using a mass to match the acoustic impedancebetween the media reduces reflected energy and improves the efficiencyof the transmission.

FIG. 1 depicts a schematic diagram of an ultrasonic transducer deviceincluding a transducer housing 106, a piezo element 102, andimpedance-matching element 104. In some embodiments the mass andmaterials of impedance-matching element 104 are selected to match theimpedance of the piezo 102 to the tissue 120 being treated. The tissuecontains one or more hair shafts 124 and one or more hair bulbs 122. Insome embodiments, the ultrasonic transducer device is capable ofproducing a substantially collimated energy beam 110. In someembodiments, what is meant by producing a substantially collimatedenergy beam is that the beam does not vary by more than 20% over adistance of at least 5 mm from the surface of the skin tissue to atreatment plane. In some embodiments, what is meant by producing asubstantially collimated energy beam is that the beam does not vary bymore than 10% over a distance of at least 5 mm from the surface of theskin tissue to a treatment plane. In other embodiments, what is meant byproducing a substantially collimated energy beam is that the beam doesnot vary by more than 5% over a distance of at least 5 mm from thesurface of the skin tissue to a treatment plane.

In some embodiments, a chill plate is used to cool the tissue 120 beingtreated. In some embodiments the impedance-matching element 104 alsoacts as a chill plate, reducing the complexity of the device. Awell-designed impedance-matching chill plate should posses the followingproperties: (1) one or more layers of material whose thicknesses andacoustic impedances are chosen to maximize the transmission of acousticenergy into the skin; and (2) good thermal conductivity in order toextract heat from the tissue for efficient cooling.

In a preferred embodiment, the impedance-matching chill plate consistsof one or more layers whose thickness and acoustic impedances are chosensuch that at least 50% of the ultrasonic energy produced by the piezo102 is transmitted to the skin tissue 120. In some embodiments, theimpedance-matching chill plate consists of one or more layers whosethickness and acoustic impedances are chosen such that at least 75% ofthe ultrasonic energy produced by the piezo 102 is transmitted to theskin tissue 120.

Materials suitable for a chill plate include, for example, aluminum,copper, brass, glass, fused silica, sapphire, and epoxy. A material withthermal conductivity comparable to or better than one of these materialsis preferred. In some embodiments the thermal conductivity is equal toor greater than 1 W/m K. In some embodiments, the impedance-matchingchill plate may be mounted to a temperature-regulated housing that actsas a heat sink.

A third consideration when using a transducer producing a broad,collimated beam is that a uniform energy distribution is produced in thehair follicles being irradiated. When using a broad, collimated beam,the energy profile is typically irregular in portions of the beam closeto the source. This portion of the beam is also referred to as thenear-field. While the beam area and average energy intensity across thebeam are relatively constant through the near-field, the energy profilemay exhibit strong oscillations due to interference effects. As aresult, there will be zones of relatively low intensity and zones ofrelatively high intensity. Hairs located in low-intensity zones may beunder-treated and their ability to regenerate unaffected.

FIG. 2 depicts an example of a near-field intensity profile 202 producedusing a 10 MHz source and a 2-mm-wide aperture. The aperture half-width(a) may be defined as the aperture width/2. The profile is calculated ata distance of 2 mm from the aperture plane. As shown in FIG. 2, thereare multiple zones of low intensity 204 approximately 0.5 mm wide. Thewidth of a hair structure is typically less than 0.25 mm and the lengthof the bulb and bulge of a hair follicle are about 0.5 mm or less. Atransducer with an intensity profile similar to the profile depicted inFIG. 2 may under-treat hairs located in low-energy zones of the beam.

The near-field may be defined as the region of the beam where thedistance (z) from the active element is less than the Rayleigh range(z_(R)). The Rayleigh range is defined by the area (A) of the clearaperture of the transducer and the wavelength of the beam (X) accordingto the equation:

Z _(R) ≡A/λ.   (Equation 1)

For example, for a 0.5-cm² aperture and 10 MHz ultrasound frequency, thenear-field would extend to 33 cm—well beyond the face of the transducer.Therefore, simply placing the hair follicle (target) in the far-field ofthe transducer is not a practical solution. The following sectiondiscusses a number of embodiments directed toward eliminating orreducing the magnitude of these oscillations.

2. Aperture Size for Optimizing a Beam's Intensity Profile

There are a number of potential sources of near-field interferenceeffects. In particular, the edges of a transducer's clear apertureproduce edge waves that interfere with the wave transmittedgeometrically through the clear aperture. This effect may create theripples observed in the profile shown in FIG. 2. In practice, the clearaperture of the transducer may be determined, at least in part, by theaperture of the active area of the active element or by some otherlimiting aperture of the transducer device that defines the beam size,such as the housing or the impedance-matching element.

Reducing the amplitude and width of the ripples is desirable when thebeam is used for hair removal. In one embodiment, the amplitude andwidth of the ripples may be reduced by increasing the width of the clearaperture of the transducer. The number of large scale ripples in thenear-field may correlate to the Fresnel number (N), which is defined bythe wavelength (X), aperture half-width (or radius) (a), and distancefrom the aperture (z) by:

N≡a ² /λz.   (Equation 2)

Since the number of ripples increases with the square of the apertureradius (a), the width of the ripples will decrease linearly withincreasing aperture size.

FIG. 3 depicts a plot of intensity profiles 302, 304, and 320 foraperture widths of 2, 4, and 20 mm, respectively. The intensity profilesare normalized as x/a, where x is the radial distance from the center ofthe aperture and the half-width (radius) of the aperture is a. As shownin FIG. 3, the amplitude of the ripples, especially in the centralportion of the beam, is reduced dramatically as the width of theaperture is increased. Furthermore, as the aperture is increased, thespatial frequency increases, reducing the width of the “holes” in theintensity profile.

For most medical applications, the damage to the targeted tissue—andtherefore the efficacy of the treatment—increases monotonically with theintensity of the applied radiation. Consequently, the optimum intensityis generally the maximum intensity that may be used without exceeding athreshold level that produces adverse side effects. A preferredintensity profile is a top-hat shape (for which the intensity issubstantially constant) because the intensity may be set to an optimumvalue across the entire beam cross-section and treatment plane.

As seen in FIG. 3, another benefit of using a larger aperture for theultrasound transducer is that the intensity profile approaches the idealtop-hat shape. Less energy is wasted in low-intensity tails 308, asobserved at x/a>1 for the smaller aperture size, and the central portionof the beam approaches a constant intensity except at the edges. Thehigh intensity at the edges may be attenuated using apodization (see,e.g., FIG. 10). Compare the intensity profiles of FIGS. 3 and 10 withthat of a focused device, as shown in FIG. 4. A focused device may havean intensity profile close to the focal plane characterized by thewell-known Airy pattern. As shown in FIG. 4, the intensity changesdramatically across the beam profile and, therefore, only a smallfraction of the beam (e.g., a narrow region at the peak) may be set tothe optimum intensity. In general, the beam profile will approach theAiry pattern for Fresnel numbers<0.5.

FIG. 5 compares the energy density versus normalized intensity for fourultrasonic beams, where the normalized intensity is the intensitydivided by the maximum intensity I_(max). Intensity distribution 504represents the normalized intensity of a collimated beam, 2 mm from a4-mm-wide aperture. The intensity distribution 504 may correspond to theintensity profile 304, as shown in FIG. 3. Intensity distribution 520represents the normalized intensity of a collimated beam, 2 mm from a20-mm-wide aperture. The intensity distribution 520 may correspond tothe intensity profile 320, as shown in FIG. 3. Intensity distribution522 represents the normalized intensity of a collimated beam, 2 mm froma 20-mm-wide, apodized aperture. The intensity distribution 522 maycorrespond to the intensity profile 1002, as shown in FIG. 10. Intensitydistribution 508 represents the normalized intensity of a focused beamclose to the focal plane. The intensity distribution 508 may correspondto a focused intensity profile, such as the Airy pattern shown in FIG.4.

As shown in FIG. 5, a narrower intensity distribution may be achieved bylocating the treatment plane in the near-field of a wide limitingaperture (i.e., at high Fresnel number). For a 4-mm-wide aperture, theintensity distribution 504 is concentrated between 65 and 85% of themaximum intensity. For a 20-mm-wide aperture, the intensity distribution520 is narrower and is concentrated between 70 and 75% of the maximumintensity. By adding apodization to a 20 mm aperture, as shown inintensity profile 1002 (FIG. 10 below), the intensity distribution 522is further narrowed to 94 to 96% of the maximum intensity. Incomparison, the intensity distribution 508 for the focused beam ispoorly concentrated and spans, relatively evenly, from 5 to 100%. Infact, the shape of the intensity profile for a focused beam isindependent of the beam size. Therefore, the intensity distribution 508cannot be made narrower by simply adjusting the size of the beam.

As described above, it is desirable to produce a beam that has a top-hatshape and to minimize the amplitude and maximize the spatial frequencyof any ripples in the profile. In some embodiments, the quality of thebeam's intensity profile scales with the Fresnel number (N) defined inEquation 2. For the 2-, 4-, and 20-mm-wide aperture intensity profiles(302, 304, 320) shown in FIG. 3, the corresponding Fresnel numbers (N)are 3, 13, and 333, respectively. For the purpose of hair removal, aFresnel number greater than 3 and more preferably greater than 10 isdesirable, calculated using distance (z) as the distance from the clearaperture of the device to the target tissue (e.g., depth of the hairfollicles). The distance (z) may also be expressed as the distance fromthe limiting aperture (e.g., face of transmitting piezo) to a treatmentplane. For hair removal applications, the treatment plane is the averagedepth of the hair follicles in the skin tissue.

3. Aperture Shape for Optimizing a Beam's Intensity Profile

The intensity profiles shown in, for example, FIGS. 2 and 3 werecalculated for a 1-dimensional slit. While the improvements in beamquality with Fresnel number noted above also apply generally to2-dimensional apertures of arbitrary shape, there are additionalconsiderations that arise when considering 2-dimensional apertures. Fora rectangular aperture, the acoustic field will simply be theconvolution of the fields produced by two 1-dimensional slits rotated by90 degrees. Therefore, the intensity profiles produced alongcross-sections parallel to the edges of the aperture will be similar tothose shown in FIG. 2 and FIG. 3.

However, for a non-rectangular aperture, additional ripples in theprofile may be observed. For example, this effect may be observed for acircular aperture which may have a strong spike or hole in the intensityprofile, on-axis with the center of the transducer. FIGS. 6 a and 6 bshow a comparison of intensity profiles for 2-mm-wide square andcircular apertures, respectively. The on-axis intensity for the squareaperture is relatively even, while for the circle it oscillates between0 and about 4 times the average intensity. If a hair follicle werelocated in the hole of the circular aperture, it would be under-treated.Furthermore, the spike could also result in over-treatment or damage tothe skin. In fact, the peak amplitude of the on-axis oscillation for acircular aperture (in the near-field) is independent of the Fresnelnumber and therefore cannot be reduced by changing the nominal size ofthe clear aperture.

In general, there are 2 advantages for using a rectangular (or square)aperture for hair removal. First, it provides better on-axis beamquality with higher efficacy and less risk of overexposure. Second, witha rectangular aperture it is easier to treat an extended area at a fixeddosimetry since the area may be neatly tiled into rows and columnswithout gaps or overlap between treatment spots.

In practice, modest deviations from a circular shape would also improvethe beam quality. Examples include an ellipse or round shape withirregular edges, as shown in FIG. 7 a and FIG. 7 b. In some embodiments,the aperture has a serrated, rippled, notched, or jagged edge profile.

4. An Apodized Aperture for Optimizing a Beam's Intensity Profile

As discussed above, a transducer design with a large Fresnel number maybe used to produce a beam with an improved intensity profile. In someembodiments, an improved profile is a profile that approaches atop-hat-like shape and reduces the amplitude of the ripples across thecenter of the beam. However, as illustrated in FIG. 3, the improvedintensity profile (at least in the near-field) still has peaks at theedges that are not attenuated by increasing the Fresnel number. Forapplications such as hair removal, it is desirable to produce a beamwith a uniform intensity profile such that the intended dosimetry may bedelivered across the entire beam. One method to reduce the edge effectsis to soften or “apodize” the edges of the limiting aperture byeliminating the single-step discontinuity in the aperture transmissionat the edge. For example, a multi-step or continuous transmissiongradient may be used.

As an example, FIG. 8 depicts an embodiment including a transducer forwhich the active element is an air-backed piezo disk 802 coupled to animpedance-matching plate 804 to produce an acoustic beam 806. Anapodizing annulus 810 is coupled to the piezo disk 802. In thisembodiment the edges of the piezo disk 802 define the clear aperture ofthe device. An AC voltage element 808 is used to drive the piezo disk802 in a piston mode such that it vibrates along the y-axis. As thepiezo 802 vibrates, an acoustic wave is generated at the piezo-airinterface. Due to the large acoustic impedance discontinuity at the airinterface, acoustic waves are strongly reflected at the transition andpropagate back through the impedance-matching plate 804. Withoutapodization, the reflection will be nearly 100% across the air interfaceof the piezo up to the edges.

As shown in FIG. 8, an apodizing annulus 810 may be added to reduce thereflection at the air interface by causing a stepped or gradedreflection. FIGS. 9 and 10 illustrate exemplary effects created using astepped or graded apodizing element. FIG. 9 depicts the reflectedintensity across an air-piezo interface for a piezo with an apertureradius (a). The reflection profile 902 represents an air-piezo interfacewithout apodization. The reflection profile 904 represents an air-piezointerface with a stepped apodization and the reflection profile 906represents a graded apodization. In some embodiments, the steppedapodization is achieved using an annulus, as shown in FIG. 8. In someembodiments, the graded apodization is achieved using a tapered orvariable impedance annulus.

FIG. 10 shows a comparison of the resulting intensity profiles withoutand with the graded apodization. The transducer in FIG. 10 is for a beamwith a Fresnel number of 333 using a graded apodization similar to thereflection profile 906 in FIG. 9. In particular, the intensity profile1002 is for a transducer with a graded apodization. The intensityprofile 1004 is for a transducer with no apodization. The intensityprofile 1002 demonstrates how apodization can be an effective means foreliminating the high-intensity peaks at the edges of near-field beam'sintensity profile. Using apodization, even the amplitude of the ripplestowards the center of the beam is reduced.

Whatever the physical origin of the effective aperture (the activeelement, the matching plate, etc.), the general principle of apodizingthe edges to eliminate edge effects may be applied. Generally speaking,the apodization has the property that it modifies the amplitude and orthe phase of the wave at the aperture edges in a way to eliminate asingle-step discontinuity in transmission.

For example, in some embodiments, an apodizing annulus may beconstructed using layers of materials, each layer having a slightlyhigher acoustic impedance. By minimizing the change in acousticimpedance from piezo to the air interface, the reflected waves would bereduced, resulting in a more uniform intensity profile. In otherembodiments, the apodizing annulus could be placed on the opposite sideof the piezo to attenuate the edges of the transmitted beam's intensityprofile. In other embodiments, portions of the acoustic beam could bereflected back to the active element to produce a more uniform intensityprofile. Other techniques known in the art could also be used to apodizethe edges of the transmitted beam. For example, the irregular apertureedges shown in FIG. 7 a and FIG. 7 b provide a form of apodization.

6. Using Frequency Modulation to Optimize a Beam's Intensity Profile

As described above, the near-field of an acoustic beam is susceptible tointerference from secondary waves that may be caused by abrupttransitions in the transducer medium For example, the edges of theaperture or an air-transducer interface can cause ripples in a beam'sintensity profile. Different aspects of the embodiments described abovecan be used to minimize some of these interference effects.

However, imperfections in the design or construction of the transducermay create additional transitions within in the transducer materials(e.g., piezo element, impedance-matching element, chill plate). Forexample, defects in the transducer materials or bond lines can producescattering. Also, small acoustic impedance mismatches at materialinterfaces may result in reflections that interfere with the primarybeam. Interference from these secondary waves may result in ripples orhot spots in the near-field intensity profile. In some cases, asecondary wave that contains 1% of the total beam power can produceripples in the near-field intensity that have a peak-to-peak amplitudeequal to 40% of the average intensity. As previously discussed,inconsistency in the beam intensity may result in underexposure of hairslocated in zones of low intensity, rendering the irradiation treatmentineffective for at least some of the hairs.

In some cases, it may not be practical, or even possible, to reduce suchvariations in the intensity profile using different aspects of theembodiments discussed above. Therefore, some embodiments may alsoinclude the ability to modulate the frequency of the device to reduce oreliminate variations in the effective intensity profile of thenear-field transducer beam.

In general, the locations of the maxima and minima in an interferencepattern depend on the frequency being transmitted. Further, it ispossible to shift areas of low intensity by modifying the frequency.Therefore, zones of low intensity at a first frequency may becompensated for by transmitting a second beam using a second, differentfrequency. If the frequency is modulated at a rate that is faster thanthe thermal relaxation rate of the targeted tissue, the tissue willintegrate the two intensity profiles during its thermal relaxationperiod. In some embodiments, the frequency is modulated at a rate thatis greater than 0.25% of the average frequency per 100 milliseconds.

In some embodiments, the frequency modulation can be used to produce aneffectively homogenous intensity distribution resulting in a smoothtemperature profile across the hair follicles within the transducer beamIn such a case, all of the hair follicles within the treatment area mayreceive an effective irradiation treatment without overexposure orunderexposure.

FIG. 11 a depicts an example of a thermal profile generated by a1-cm-radius transducer operated at 10.72 MHz. In this case, theintensity profile is mapped by recording the thermal image in a3-mm-thick sample of absorbing tissue mimicking material (“TMM”). Thethermal image of an irradiated TMM approximates the ultrasound intensityprofile that could be expected in a hair removal operation. As shown bythe darker portions in FIG. 11 a, there are low intensity portions ofthe irradiating energy approximately 1-mm-wide. Human hairs areapproximately 100 um in diameter and spaced about 1 mm apart in anepidermal tissue. Using the irradiation intensity profile as shown inFIG. 11 a to administer a hair reduction treatment would result in asignificant portion of hair being underexposed and, therefore,unaffected by the treatment.

FIGS. 11 b-e depict thermal profiles recorded at frequencies of 10.77,10.82, 10.87, 10.92 MHz, respectively. The figures illustrate that thefringe pattern shifts spatially with changes in the irradiationfrequency. Combining the intensity profiles of multiple irradiations,each at different frequencies, results in an effective beam profile thatis more uniform. The combined profile will tend to be smoother since theintensity maxima at one frequency tend to fill in the minima at another.

FIG. 12 shows an effective beam profile calculated by averaging thefrequencies 10.72, 10.77, 10.82, 10.87, and 10.92 MHz using a weightingfactor of 5:8:5:8:4, respectively. In practice, the multipleirradiations could be administered at each frequency for a pulseduration according to the weighting factor. In one embodiment, thefrequencies 10.72, 10.77, 10.82, 10.87, and 10.92 MHz are eachadministered at a pulse time of 5:8:5:8:4 milliseconds, respectively. Asindicated by the reduced low-intensity zones in FIG. 12, the homogeneityof the combined exposure has improved significantly. Therefore, theestimated efficacy of a hair reduction treatment using frequencymodulation would be improved relative to a single frequency treatment.

FIGS. 13 a and 13 b compare calculated combined frequency profiles witha corresponding measured (actual) frequency-modulated profile as appliedto a TMM. FIG. 13 a depicts a calculated and measured thermal profileusing two frequencies with a 2:1 weighting. FIG. 13 b depicts acalculated and measured thermal profile using six frequencies with1:1:2:7:4:3 weighting. As illustrated in FIGS. 13 a and 13 b, theexperimental results agree reasonably well with the calculation.

In some embodiments, the weighting may be realized by modulating thefrequency and controlling the dwell time at each frequency. In someembodiments, the frequency may be swept though one or more ranges offrequencies over a treatment time. In some embodiments, the one or morepulses may be separated by a pause to allow the thermal energy in thetissue to dissipate. In some embodiments, a modulation of the drivefrequency of 1% produces a significant improvement in the homogeneity ofthe exposure. Therefore, in a preferred embodiment, the frequency rangeof the drive signal should be equal to or greater than 0.25% of theaverage drive frequency. In a more preferred embodiment, the frequencyrange of the drive signal should be equal to or greater than 1% of theaverage drive frequency.

FIGS. 14 a and 14 b depict exemplary embodiments using frequencymodulation. FIG. 14 a shows a single-element device driven with afrequency-modulated waveform. The waveform may consist of multiplediscrete frequency steps, a continuous frequency sweep, or somecombination thereof. The frequency steps or sweep may be realized withina single pulse (as shown in waveforms W1 and W2) or a series of pulses.For example, W3 shows a series of 3 single-frequency pulses each havinga different frequency. In this case, the improvement in the homogeneityof the exposure is due to the cumulative effect of multiple pulses. FIG.14 b depicts another embodiment using multiple elements, each elementdriven sequentially at different frequencies. The waveforms of thedevices could also overlap in time and each waveform could consist ofmultiple discrete frequency steps, a continuous frequency sweep, or somecombination thereof. In addition, a variety of 3-dimensional geometricarrangements of the elements can be envisioned that would overlap thebeams from each element on the target.

While the discussion above has focused on the spatial variation of thefield in the transverse direction, the field will also vary rapidly withlongitudinal distance from the transducer. A calculation of the fieldintensity for an exemplary 1-cm-radius transducer driven at 10 MHz isshown in FIG. 15.

As shown in FIG. 15, the intensity oscillates with a spatial period ofabout 200 um Therefore, a target whose size scale in the longitudinaldirection is on the order of 100 um might avoid exposure if the targetwere located close to a minimum in the longitudinal intensity profile.Typically, hair follicles are approximately 100 um in the longitudinaldirection and their depth relative to the skin surface may vary byseveral 100 um Therefore, exposure by a single frequency may not beeffective since it will not affect the growth of hairs whose bulbs orfollicles are located close to intensity minima However, as was observedfor the profiles in the transverse plane, the longitudinal intensityprofile can also be shifted spatially by adjusting the drive frequency.FIG. 15 shows that shifting the frequency to 10.085 MHz results in anintensity profile shifted by half a spatial period with respect to 10.0MHz. Therefore, the same concepts of frequency modulation and devicedesign discussed above in relation to the exposure in the transverseplane, may also be employed to improve the efficacy in cases where thetarget size is comparable to or smaller than the spatial period of theintensity in the longitudinal direction.

7. Techniques for Imaging a Transmitted Beam Without Affecting IntensityProfile

In some cases, practical requirements or other physical constraintsrequire the device to separate the transducer from the target by somedistance. If the beam traverses this distance by simple free-spacepropagation then, according to Equation 2, the Fresnel number woulddecrease and therefore the beam quality would deteriorate. However, thislimitation may be easily overcome by imaging the near-field intensityprofile to a distant plane using acoustic lenses or mirrors. Forexample, if an acoustic lens of focal length (f) is placed a distance s1from the near-field intensity profile, the near-field image will bereproduced at a distance equal to s2 on the opposite site of the lens.The parameters f, s1, and s2 are related by:

$\begin{matrix}{\frac{1}{f} = {\frac{1}{s\; 1} + {\frac{1}{s\; 2}.}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

FIG. 16 depicts an example of imaging a beam using an acoustic lens 602.Item 606 represents a near-field beam profile produced using transducer604 located a distance s1 from the imaging lens 602. Item 608 representsan image of the near-field profile projected a distance (s2) from theimaging lens 602. In other embodiments, a combination of lenses andminors, placed at appropriate locations, may be used to image thenear-field and achieve a desired magnification, working distance, orphysical distance between the object and image.

8. Exemplary System Using an Ultrasonic Transducer

FIG. 17 depicts an exemplary system 1700 using an ultrasonic transducerdevice for hair removal in a dermis tissue layer 1720. In oneembodiment, one or more transducer elements 1710 are used to irradiatethe tissue layer 1720 with an energy beam. In some embodiments, multipletransducer elements 1710 may be contained in one device housing. In someembodiments, more than one transducer elements 1710 may be stacked oraligned along a transmitting axis.

A frequency generator 1704 is used to produce the excitation voltage forthe one or more transducer elements 1710. The frequency generator 1704may be any waveform generation device suitable for delivering anultrasonic frequency voltage to the one or more piezo elements used inthe one or more transducer elements 1710. In some embodiments, more thanone waveform-generation device is used as the frequency generator 1704.In some embodiments, the frequency generator 1704 may be controlled by acomputer controller 1702. In some embodiments, the frequency generator1704 includes an internal controller in addition to, or instead of,computer controller 1702. In a preferred embodiment, it is possible toset the frequency generator 1704 to more than one excitation frequencyand more than one pulse time.

The computer controller 1702 may include one or more processors forexecuting computer-readable instructions. The computer-readableinstructions allow the computer to control the frequency generator 1704to produce one or more pulse frequencies at one or more pulse times. Thecomputer controller may also include computer memory, such as read-onlymemory (ROM), random-access memory (RAM), and one or more non-volatilestorage media drives for storing computer-readable instructions orprograms. The computer controller may be equipped with a computerdisplay 1706 or other visual read-out device.

It should be appreciated that the various features of the embodimentsthat have been described may be combined in various ways to producenumerous additional embodiments. Accordingly, the invention is not to belimited by those specific embodiments and methods described herein.

1. An ultrasonic transducer system for treating tissue comprising: afrequency generator for generating an AC voltage; a unitary transducerreceiving the AC voltage from the frequency generator for producing anultrasonic energy pulse at an ultrasonic frequency for a pulse width;and a transmitter element coupled to the transducer for irradiating aportion of skin tissue, the transmitter element having an acousticaperture for producing a substantially collimated energy beam, the beamhaving a width greater than 4 mm wherein the cross-sectional area of thesubstantially collimated energy beam does not vary by more than 20% overa distance of at least 5 mm from the surface of the skin tissue to atreatment plane and wherein the substantially collimated energy beam isproduced by a single transducer.
 2. The ultrasonic transducer system ofclaim 1 further including a chilled surface in contact with the skintissue.
 3. The ultrasonic transducer system of claim 1, wherein theintensity of the ultrasonic energy pulse at the transducer is greaterthan 150 W/cm².
 4. The ultrasonic transducer system of claim 1, whereinthe pulse width is less than 100 milliseconds.
 5. The ultrasonictransducer system of claim 1, wherein the ultrasonic frequency isbetween 5 and 20 MHz.
 6. The ultrasonic transducer system of claim 1,wherein the ultrasonic energy pulse has an acoustic wavelength, and thesquare of half the acoustic aperture, divided by the product of theacoustic wavelength and the distance from the acoustic aperture to 5 mmbelow the skin surface, is greater than
 3. 7. The ultrasonic transducersystem of claim 1, wherein the ultrasonic energy pulse has an acousticwavelength, and the square of half the acoustic aperture, divided by theproduct of the acoustic wavelength and the distance from the acousticaperture to 5 mm below the skin surface, is greater than
 10. 8. Theultrasonic transducer system of claim 1, wherein the Fresnel number ofthe beam is greater than
 3. 9. The ultrasonic transducer system of claim1, wherein the Fresnel number of the beam is greater than
 10. 10. Theultrasonic transducer system of claim 1, wherein the substantiallycollimated energy beam has a cross sectional area at least 16 mm² fromthe surface of the skin tissue to 5 mm below the surface of the of skintissue.
 11. The ultrasonic transducer system of claim 1, wherein theintensity of the ultrasonic energy pulse is greater than or equal to 150W/cm, the ultrasonic frequency is between 5 and 20 MHz, and the pulsewidth is less than 100 milliseconds.
 12. The ultrasonic transducersystem of claim 1, wherein the acoustic aperture has a non-circularshape.
 13. The ultrasonic transducer system of claim 1, wherein thetransducer produces a plurality of ultrasonic energy pulses, each energypulse at a different frequency and pulse width.
 14. The ultrasonictransducer system of claim 1, wherein the transducer produces aplurality of ultrasonic energy pulses and the difference between ahighest pulse frequency and a lowest pulse frequency is greater than0.25% of an average frequency of the plurality of ultrasonic energypulses.
 15. The ultrasonic transducer system of claim 1, wherein thetransducer produces a pulse that is continuously swept over a range offrequencies.
 16. The ultrasonic transducer system of claim 1, whereinthe transducer produces an ultrasonic frequency that is swept at a rategreater than or equal to 0.25% of the average frequency per 100milliseconds.
 17. The ultrasonic transducer system of claim 1, whereinthe edge of the acoustic aperture is apodized to attenuate thetransmitted power near a perimeter of the energy beam.
 18. Theultrasonic transducer system of claim 17, wherein the transmitterelement includes an apodizing annulus for attenuating the transmittedpower near the perimeter of the energy beam.
 19. A method of inhibitinghair growth in a skin tissue, the method comprising: placing a unitaryultrasonic transducer in contact with the skin tissue, the transducerhaving an acoustic aperture for producing a substantially collimatedenergy beam over a treatment area greater than 16 mm², wherein thesubstantially collimated energy beam is produced by a single transducer,herein the cross-sectional area of the substantially collimated energybeam does not vary by more than 20% over a distance of at least 5 mmfrom the surface of the skin tissue to a treatment plane, the tissuehaving a plurality of hair follicles within the treatment area; andgenerating one or more ultrasonic energy pulses, each energy pulse at acorresponding frequency and pulse width.
 20. The method of claim 19further including the step of cooling the tissue surface.
 21. The methodof claim 19, wherein the frequency of the ultrasonic energy is varied.22. The method of claim 21 wherein the ultrasonic frequency is swept ata rate greater than or equal to 0.25% of the average frequency per 100milliseconds.
 23. The method as recited in claim 21 wherein thevariation between the highest and lowest frequencies of the ultrasonicenergy is at least 1.0% of the average drive frequency.
 24. The methodof claim 19, wherein the combined pulse widths of all of the energypulses total less than 100 ms.
 25. The method if claim 19, wherein theintensity of the ultrasonic energy pulse at the transducer is greaterthan 150 W/cm².
 26. The method of claim 25, wherein the frequency of theultrasonic energy pulse is between 5 and 20 MHz.